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I don't like the sand, but apparently deserts are inevitable since I don't remember reading about a period on earth when there were no deserts.

So I was thinking, what invasive plants of earth has the potential to turn a dry and hot desert into something greener? or do I have to invent some mega-super-duper-uber-pflanze like the ''handwavia Fabacea'' ?

But we have stuff already growing on vulcanoes or antartica so I probably there must be something that can colonize deserts like the sahara that just didn't have a chance to occur yet.

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    $\begingroup$ The desert is there because lack of water, either atmospheric or terrain-based issues deprive a region of water, and there you go, a desert. You would probably need cactuses like those in US's dry regions that would be able to absorb and preserve water inside themselves in order to help other life survive nearby, but if you just don't have a source of water, ANYWHERE, nothing could help. $\endgroup$
    – Vesper
    Commented Jan 19, 2023 at 10:50
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    $\begingroup$ -1 for lack of research. Had humanity an answer to this question as asked, we would have implemented it thousands of years ago. You are expected to do your due diligence before asking questions. You've not researched why deserts are the way they are - and it's a LOT more complicated than lack of water. Utah's Strawberry Reservoir is surrounded by what is almost a desert (sparse sage brush). Lack of water isn't the problem. Lack of soil/compost, supporting bacteria, and the supporting insects and animals to sustain greater plant growth are all involved. $\endgroup$
    – JBH
    Commented Jan 19, 2023 at 11:08
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    $\begingroup$ And I'd like to make one more comment. Over the last few years there has been a growing tendency to ask for help developing a completely fantastic condition as if it could really exist. (See this Meta question expressing frustration over the issue). Per the help center, we're here to help you build an imaginary world. Let's stick to that context. What's stopping you from setting a rule that you have a desert-condition plant that's prolific? $\endgroup$
    – JBH
    Commented Jan 19, 2023 at 11:17
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    $\begingroup$ @JBH sounds like some stupid logic ''if there was something we would have already solved the problem''..... no it doesn't work like that sometimes the solution is in front of your nose but can't be used or won't be used either because of politics, environmentalists moralists or because people just didn't notice it. $\endgroup$
    – user100394
    Commented Jan 19, 2023 at 11:41
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    $\begingroup$ Nice Try Muad'dib - stop trying to fulfil the prophecy and make a paradise of Arrakis. $\endgroup$ Commented Jan 19, 2023 at 19:48

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I think your question is a bit confused. Let's look at the different parts:

I don't like the sand, but apparently deserts are inevitable since I don't remember reading about a period on earth when there were no deserts.

OK, Earth has always had sand deserts. I'll take your word for it but I guess it isn't implausible.

So I was thinking, what invasive plants of earth has the potential to turn a dry and hot desert into something greener?

If one did, why would there still be deserts on Earth ? Why would there always have been sand deserts on Earth ? That's the thing about life - it tends to go everywhere it can, especially given geological timescales to work with.

But we have stuff already growing on vulcanoes or antartica so I probably there must be something that can colonize deserts like the sahara that just didn't have a chance to occur yet.

I mean, there are stuff growing on volcanoes and Antarctica yet Antarctica has very few stuff growing in it. I'm less sure why you're highlighting volcanoes as being inhospitable, when they're not erupting they're a mountain like any other and pretty fertile as those go actually IIRC, and when they're erupting, well... I don't know of any stuff that grows in liquid lava. Either way, there's also stuff growing in the Sahara. The issue is there's not a lot of it, but that's also true of Antarctica.

But either way the same issue arises as with the previous sentence - if such a thing existed it would have greened the Sahara already, would it not? So logically it probably doesn't exist in our current world, right?

or do I have to invent some mega-super-duper-uber-pflanze like the ''handwavia Fabacea'' ?

That seems like the obvious conclusion and I'm not sure what other answer you're expecting. Are you asking for general principles of plant life to help you construct a plausible ''handwavia Fabacea'' ? Or do you think there is an actual plant species out there that could green all deserts but didn't for lack of opportunity, and somebody on this forum knows about this plant, knows it could green all deserts but hasn't done anything to spread this knowledge or act on it ? (not saying they should act on it if they knew, just that humans being what they are it seems pretty surprising they wouldn't at least have tried something, or talked about it. More to the point even acquiring the knowledge that a plant could, but didn't, green the Sahara would IRL require experimenting with actually making the plant do that)

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Everybody who has ever studied permaculture has learned Bill Mollison's answer that American deserts have cacti, African deserts have bulbs, Australian deserts have seed-foods, and by combining them in one desert, you could get a high food-yield.

The Kalahari has 16 nice berries, but the best food is below the ground. The Marama bean is an important desert food in the Kalahari. You've got the marotse melon, you've got over 400 cultivated cacti in America, including dragonfruit and prickly pear. Here is a wonderful 2000-year-old food forest planted and grown Morocco. It has: date palms, olives, figs, guava, citrus, mulberry, tamarind, carib, pomegranate, banana, quince, grape.

The key to turning the desert green is reducing evaporation. (Others have commented that deserts are there because there's no water whatsoever. This is wrong. There's water in all places. There's precipitation in all places except for the Atacama, and that's foggy.)

There are lots of desert trees from different continents, and by assembling them, you start to trap moisture. I remember once seeing a list of TEN ways in which trees improve moisture. I don't remember them all, but some are: the humus they put in the ground improves the water retention, orographic effects create rain, shade reduces evaporation, leaves act as a surface for condensation, etc.

You didn't specify which desert you are talking about – growing conditions in Australia are very different from in the Lut Desert.

Taking the Sahara as one example, there are over 1.8 billion trees in an area of 1.3 million km². The study notes "The canopy cover increases from 0.1% (0.7 trees per hectare) in hyper-arid areas, through 1.6% (9.9 trees per hectare) in arid and 5.6% (30.1 trees per hectare) in semi-arid zones, to 13.3% (47 trees per hectare) in sub-humid areas", with the definitions "hyper-arid (rainfall of 0–150 mm yr−1), arid (rainfall of 150–300 mm yr−1), semi-arid (rainfall of 300–600 mm yr−1) to sub-humid (rainfall of 600–1,000 mm yr−1) areas. Britannica says of the Sahara, "Although precipitation is highly variable, it averages about 3 inches (76 millimetres) per year.", so the majority of it you could expect 0.7 trees per hectare to already be there naturally, and you'd likely have good luck growing more, especially if you use the best techniques like clay pots buried near the sapling, mulch, etc.

Point is there are trees in deserts, as anyone who has been in deserts knows. Deserts are just places where precipitation is low and evaporation is high. The best way to reduce evaporation is trees. You can assemble desert trees from different continents: Africa's Ricinodendron rautanenii (Manketti or mongongo), the Quandong tree from Australia, various acacias, various cypresses, the Mesquite tree which grows in salt pans and provides forage for bees as well as herbivores. A lot depends on the specifics of your desert (salinity, alkalinity, precipitation, other xericulture techniques like earthworks), but your question specified plants and these are the ones you want.

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  • $\begingroup$ Sure there is some precipitation in the Sahara. About 100 to 150 mm (4 to 6 in) per year. Good luck growing a tree with that. As for the mysterious Mr. Mollison, do you have a link to a description of the wonderful gardens he planted and grew in the great Saharan hamadas? Oh wait, he didn't. $\endgroup$
    – AlexP
    Commented Jan 19, 2023 at 13:31
  • $\begingroup$ The trouble isn't growing a tree with it... it's growing anything at all such that the sand will stop duning up, soil can be built for the next 10,000 years and trap enough moisture that you can bootstrap up to better plants. The word "desert" might not be the best one, @AlexP, but there is a problem with soil erosion in these places that makes them even more inhospitable than they'd be given their rainfall. Organisms alter their own environment to better suit themselves, unless the environment falls into a trap so extreme none can wedge in a toehold. $\endgroup$
    – John O
    Commented Jan 19, 2023 at 15:53
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    $\begingroup$ Exactly John O, bootstrapping is the word. More trees mean more moisture which makes it easier to grow even more trees. We don't know what conditions the world in the question is working with. With enough labour, organisation, etc. it's certainly possible. With a runaway massive invasion of acacias, cypreses, quandongs, and mesquites??? A little more questionable but I'd say acceptably plausible for fiction. $\endgroup$
    – wokopa
    Commented Jan 19, 2023 at 15:59
  • $\begingroup$ To develop the 'bootstrapping' idea: this means geographical bootstrapping too. The semi-arid ridges (equivalent of the Sahel) could be tree'd up more easily. This would make the desert shrink; what was deeper desert is now the new semi-arid fringe. Plant that up. Repeat. Green marches mile by mile into yellow. $\endgroup$
    – wokopa
    Commented Jan 19, 2023 at 23:25
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    $\begingroup$ @John there seem to be plants specialized to live in arid regions, that don't expend water into the air as hard as agricultural species do. They also have a better use of fog and precipitation when it comes, thus initially planting agave and cacti could work, then trees of the same kind could also start to grow by the virtue of raising local water levels and/or lowering ambient temperature. $\endgroup$
    – Vesper
    Commented Jan 20, 2023 at 8:22

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